1. Field of Invention
The present invention relates to a method for leaching of gold for recovering the gold from sulfide ores and silicate ores containing gold. More particularly, the present invention relates to the leaching method of gold in an aqueous solution under atmospheric pressure and at a temperature lower than the boiling temperature.
2. Background Technique
Copper sulfide ores and silicate ores, which are accompanied with the copper sulfide ores, contain gold. Methods for recovering the gold from such ores are largely classified into two types. One of the methods involves heating to high temperature of 1000° C. or more. Specifically, the copper sulfide ores and the silicate ores containing gold are melted together with iron sulfide. Silicate ore may be added as a flux agent. As a result, Cu2S referred to as the matte and slag mainly composed of iron oxide and silicate and containing impurities are formed. The matte is reduced at high temperature to yield low-purity metallic copper, referred to as crude copper. The crude copper is subjected to the electrolytic decomposition to yield the metallic copper having purity of 99.99% or more. Gold contained in the raw material behaves in the same manner as the copper during the production of metallic copper. Gold and the other precious metals are recovered in the precipitates referred to as “electrolytic copper sludge” during the electrolytic decomposition process.
The electrolytic copper sludge is melted together with lead so as to concentrate the precious metals in the lead. The lead, in which the precious metals are concentrated, is oxidized to remove the lead. A silver plate containing these precious metals, such as gold, referred to as a prototype silver plate, is produced. The prototype silver plate is subjected to electrolytic decomposition to electrolytically deposit the refined silver and to yield the precipitates referred to as the electrolytic silver sludge, which contains the precious metals. These precipitates are treated by nitric acid or the like to dissolve the precious metals other than gold. The resultant solid gold is melted to produce a prototype gold plate, which is gold having low purity. The prototype gold plate is subjected to electrolytic decomposition to recover the high-purity gold. High temperature exceeding 1000° C. is necessary in this method. Since the main purpose of this method is to produce metallic copper, and gold is recovered as a byproduct, the treating process until the gold recovery is disadvantageously very long.
The other method of recovering gold is to bring the copper sulfide ore or the like into contact with a solution which contains a complex-forming compound, such as cyanide, thiourea, and thiosulfuric acid, which are liable to form a gold compound. These complex-forming compounds are caused to react with the gold and the reacted gold is then dissolved in the solution. The dissolved gold is adsorbed on the surface of activated carbon. High temperature is not necessary and the recovering process is short in this process. However, when such metallic elements as copper and iron are co-present with gold, these metallic elements form complex compound, which uses up the complex-forming agents. It is, therefore, advisable to preliminarily remove the metallic elements other than gold to an appropriate level. “REVIEW OF GOLD EXTRACTION FROM ORES”, S. R. La BROOY, H. G. LINGE and G. S. Walker, Minerals Engineering, Vol. 7, No. 10, pp 1231-1241 proposes separate treatments of the gold and the metals other than the gold.
Gold and a halogen-group element form a complex compound, which is liable to dissolve in the aqueous solution. This property is utilized, for example, in U.S. Pat. No. 5,487,819, which proposes to leach gold in an aqueous solution containing a halogen-group element. An oxidizing reagent is necessary in this process to oxidize the metallic gold to the gold ions. Usually, nitric acid, hydrogen peroxide, or chlorine having the standard oxidation reduction potential of more than +900 mV are used as the oxidizing element. Since almost all of the metallic elements contained in the ore are oxidized under the presence of the oxidizing agent having the high oxidation-reduction potential mentioned above, more of oxidizing agent is consumed for oxidizing the metallic elements other than gold than that for recovering oxidizing gold (c.f., “Establishment of Wet Processing Technique for Copper Sludge”, Akinori, TORAIWA, Yoshifumi, ABE, “Shigen to Sozai” Vol. 116 (2000), No. 6, pp 484-492.